Well, Liz can both unbirth and be unbirthed - but I don't know how literal your question is. Like the in-game Encyclopedia says, unbirth is the only type of vore that isn't dangerous to the prey. There's no, like, age regression or any of the wackiness you sometimes see associated with unbirth.
Not forever, no - unbirth is more or less an advanced form of sex, although there are various scenarios where it can end up being menacing (re-birthing poor Liz right down a predator's gullet!) or lifesaving (our car broke down in a winter storm, and we need to conserve heat somehow...).
